1. Field of the Invention
The present invention relates generally to control devices and control method for friction engagement elements and particularly to the technology applied to control a friction engagement element provided internal to a fluid coupling that couples a power source and a transmission together to allow the fluid coupling to have its input and output shafts mechanically coupled together.
2. Description of the Background Art
Conventionally some transmissions (automatic transmissions in particular) are coupled to an engine via a torque converter or a similar fluid coupling. The torque converter receives a driving force from the engine and transmits the driving force to the transmission via oil or a similar fluid circulating through the torque converter. This provides the torque converter's input and output shafts with different numbers of revolutions (or speeds), respectively. In such case, the driving force can be transmitted inefficiently. Accordingly, a lockup clutch or a similar friction engagement element is provided to allow the torque converter to have its input and output shafts mechanically coupled together.
Japanese Patent Laying-Open No. 07-180766 discloses a device controlling force for engagement for a fluid coupling (a torque converter) provided with a lockup clutch. As described in the publication, the device controls the force for engagement for the fluid coupling for a transmission that is provided with a rotative driving member rotatably driven by an engine, a rotative driven member rotatively driven by the rotative driving member hydraulically, a lockup clutch capable of engaging the rotative members together by a force for engagement depending on a differential fluid pressure exerted, and a control unit controlling the force engaging the lockup clutch for an operation state falling within a predetermined range subject to controlling the force for engagement. This device includes a transmission delay unit. When the operation state is no longer within the predetermined range and the control unit withdraws from controlling the force engaging the lockup clutch, and successively there arises a condition requiring transmission, then the transmission delay unit delays the start of the transmission by a predetermined period of time.
A torque converter transmits a driving force by automatic transmission fluid (ATF) flowing between a pump impeller connected to the input shaft and a turbine runner connected to the output shaft. Accordingly, how the pump impeller and the turbine runner rotate can determine in which direction in the torque converter the ATF flows. Herein, in general, a lockup clutch is provided between the converter's cover and the turbine runner and engaged with the converter's cover to allow the torque converter to have its input and output shafts directly connected together. Accordingly, if the ATF flows from the pump impeller toward the turbine runner, it will cause a force acting in a direction to push the lockup clutch against the converter's cover (or increase the force for engagement). In contrast, if the ATF flows from the turbine runner toward the pump impeller, it will cause a force acting in a direction to pull the lockup clutch away from the converter's cover (or decrease the force for engagement). Accordingly for example if the engagement of the lockup clutch is being controlled (i.e., the lockup clutch transitions from disengagement to engagement), and the flow of the ATF varies in direction from the direction allowing the force pulling the lockup clutch away from the converter's cover to act to the direction allowing the force pushing the lockup clutch against the converter's cover to act, the lockup clutch can rapidly be engaged and hence cause shock resulting in poor drivability. Japanese Patent Laying-Open No. 07-180766, however, describes the device without considering such shock caused as the ATF flows in a varied direction.